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Toxicology Brief: Helping animals exposed to the herbicide paraquat

Last summer, an epizootic of paraquat poisoning caused the deaths of at least seven dogs in Portland, Ore.1 This epizootic is evidence that this type of poisoning remains a current problem in companion-animal practice in North America. Paraquat, a poisonous dipyridilium compound, is one of the few nonselective herbicides still available in the United States. Because paraquat is fast-acting, can be effectively used in wet environments, and has limited potential for environmental contamination and low rates of weed resistance, it is still widely used in various crop production systems. However, paraquat is highly toxic to domestic animals if ingested.2,3

This article from The Oregonian on Aug. 18, 2004, describes the discovery of meat and raw chicken that may have been planted to harm dogs in a Portland, Ore., public park. As of Sept. 2, officials ruled out paraquat poisoning in this case, and test results for other toxins are pending. Portland residents are wary because of last years incidents of paraquat poisoning in an off-leash dog park that resulted in the deaths of at least seven animals.

Sources

Within the United States, paraquat is a restricted-use herbicide with the exception of pressurized spray formulations that contain no more than 0.44% paraquat bis (methyl sulfate) and liquid fertilizer formulations that contain no more than 0.04% paraquat dichloride. Current active U.S. registered brand names include Gramoxone Super (Syngenta), Gramoxone Max (Syngenta), Cyclone Max (Syngenta Crop Protection), Marman Herbiquat Herbicide (Marman USA), and Surefire Herbicide (UAP-Loveland Products). Because paraquat has been available for agricultural use since 1962, outdated stocks are relatively easy to obtain.2 Older, outdated domestic garden herbicides often contained a 50:50 (Wt:Wt) mixture of diquat and paraquat, and supplies of this mixture can still be found in the United States. Despite paraquat's restricted-use status, intentional paraquat poisoning of animals remains a problem.3,4

Exposure and toxicokinetics

Most cases of paraquat poisoning in people and animals involve ingestion of concentrated formulations.2-8 In dogs, only about 25% to 28% of orally administered paraquat is absorbed; the remainder is excreted unchanged in the feces.2 In experiments in rodents, paraquat was detected in the feces up to seven days after exposure.2 The oral LD50 of paraquat in cats is 35 to 50 mg/kg. The oral LD50 in dogs is unknown but is higher than that in cats, and the intravenous LD50 in dogs is 7.48 mg/kg.

Many of the current commercial paraquat preparations (e.g. Gramoxone) deliberately incorporate emetics and bitters in their concentrate formulations to reduce the dose absorbed after suicide-related oral poisoning in people.2,9 This protective measure causes fasting and nonfasting dogs to vomit 61% to 86% of an orally administered dose and reduces blood paraquat concentrations by about 170 times.9 Unfortunately, older concentrates are less likely to incorporate this key safety feature.